1. Field of the Invention
The present invention relates to a method of manufacturing a vertical cavity surface emitting laser and a method of manufacturing a laser array, a vertical cavity surface emitting laser and a laser array, and an image forming apparatus with the laser array.
2. Description of the Related Art
A known configuration of a surface emitting laser is a vertical cavity surface emitting laser (VCSEL).
The surface emitting laser includes two reflectors sandwichingly holding an active region from respective sides, and a resonator formed perpendicularly to a substrate surface from which light is emitted in a vertical direction.
Each of the reflectors may be a distributed Bragg reflector (DBR) including a low refractive index layer and a high refractive index layer alternately laminated and each having an optical thickness of λ/4.
In addition to offering a stable single mode as a longitudinal mode property, the vertical cavity surface emitting laser has the excellent properties of allowing a reduction in threshold value and facilitating construction of a two-dimensional array compared to an end face emitting laser. The surface emitting laser is thus expected to be applicable as a light source for optical communication and transmission and for electrophotography.
For the vertical cavity surface emitting laser, controlling a transverse mode is an important challenge. In view of applications to communications, the transverse mode operation needs to be a single mode.
Thus, for the vertical cavity surface emitting laser, an attempt has been made to provide a single transverse mode by forming a current confinement structure inside an element by means of selective oxidation to limit an emitting region in the active layer, and at the same time, using the selectively oxidized portion to form a waveguide structure.
However, providing the single transverse mode using only the current confinement structure requires a reduction in confined diameter. The reduced confined diameter decreases the size of the emitting region. As a result, obtaining a high laser output power is difficult.
Thus, Japanese Patent Application Laid-Open No. 2000-332355 proposes a selectively oxidized vertical cavity surface emitting laser configured as follows.
An opening is formed in a top electrode formed on a top multilayer film reflector. The reflectance of a part of the multilayer film reflector which contacts the top electrode is lower than that of a part of the multilayer film reflector which is exposed in the opening.
This structure is intended to increase loss of a high-order transverse mode, in the region of the multilayer film reflector which contacts the top electrode so as to oscillate the single transverse mode based on the fundamental transverse mode.
That is, the vertical cavity surface emitting laser in Japanese Patent Application Laid-Open No. 2000-332355 is intended to control the transverse mode using the diameter of the current confinement structure and another parameter, the diameter of the opening in the top electrode, thus further stabilizing fundamental mode oscillation.
In the vertical cavity surface emitting laser in Japanese Patent Application Laid-Open No. 2000-332355, aligning a central axis of the opening in the top electrode with a central axis of a non-oxidized region of the current confinement structure is important.
Insufficient alignment between the central axes may inhibit oscillation of the fundamental transverse mode.
Thus, to solve this problem, U.S. Patent Application Publication No. 2003-0235226 proposes a method of manufacturing a surface emitting semiconductor laser as described below.
A dielectric and a metal contact layer are removed through a photo resist as a mask to form an annular etching mask formed of a metal contact layer or the like.
The inner diameter of the annular etching mask determines the diameter size of the opening in the electrode.
Then, a mesa structure is formed by RIE (Reactive Ion Etching) through the annular etching mask as a mask. A current confinement structure is provided by steam oxidation.
The diameter of the mesa structure is determined by the outer diameter of the annular etching mask. This also determines the diameter of the current confinement structure.
That is, in a photolithography stage, the inner and outer diameters of the metal contact layer are determined. The diameter of the opening in the electrode is defined by the inner diameter of the metal contact layer. The diameter of the current confinement structure is defined by the outer diameter of the metal contact layer.
With this configuration, owing to the high positional accuracy of the photolithography technique, the central axis of the electrode opening and the central axis of diameter of the current confinement structure are expected to be able to be controlled with a high positional accuracy.